1. Field of the Invention
The present invention relates to a method for press-molding a dielectric block.
2. Description of the Related Art
A known press-molding method, disclosed in Japanese Unexamined Utility Model Application Publication No. 55-71697, will be described with reference to FIGS. 17-19B. FIG. 17 shows a die 507, an upper punch 508, and a lower punch 509. The upper punch 508 is received in the die 507 as it descends (see FIG. 18), and the lower punch 509 is positioned in the die 507. The upper punch 508 and the lower punch 509 are provided with mandrels 510 and 511, respectively, which are vertically movably inserted into the upper punch 508 and the lower punch 509, respectively, at eccentric positions with respect to each other. The mandrels 510 and 511 are urged toward each other by respective springs 512 and 513.
The above molding apparatus serves to mold a dielectric block 520 (FIGS. 19A and 19B) provided with a hole 516 in which a step 515 is formed. In the molding apparatus, the mandrel 511 of the lower punch 509 is raised to a predetermined level, as shown in FIG. 17, and the upper punch 508 descends, thereby compressing a powdered dielectric ceramic 514 while the mandrel 510 is in contact with the mandrel 511, as shown in FIG. 18. Thus, the dielectric block 520 shown in FIGS. 19A and 19B is obtained.
In order to avoid cracks in the vicinity of the step 515, the powdered dielectric ceramic 514 must be compressed in such a manner that the density of ceramic to be press-molded in regions A1, A2 and A3 (FIG. 18 are the same.
However, in the known press-molding method, the position of a contact part between the vertically opposing mandrels 510 and 511 is controlled by controlling the balance of resilient forces between the springs 512 and 513 making it difficult to accurately control the position of the interface where the mandrels 510 and 511 contact one another during the pressurizing step. Therefore, a problem occurs in that the density of ceramic in the region A2 differs from that in the regions A1 and A3 and it is likely that cracks will be produced in the vicinity of the step 515.
Accordingly, it is an object of the present invention to provide a method for press-molding a dielectric block, in which the position of contact parts between upper and lower mandrels can be accurately controlled during compression, whereby cracks are not likely to be produced in the vicinity of steps of the dielectric block.
To the end, according to an aspect of the present invention, a method for press-molding a dielectric block uses a press-molding apparatus provided with an upper mold including an upper punch provided with a slidingly movable upper mandrel, and a lower mold including a die having a cavity and a lower punch provided with a slidingly movable lower mandrel. The upper punch is slidingly movable in the cavity of the die and the lower punch is coupled in the cavity of the die. The method comprises the steps of charging a predetermined amount of a powdered dielectric material into the cavity when the lower mandrel protrudes from the lower punch into the cavity; moving at least one of the upper mold and the lower mold so as to approach and come into contact with each other with a lower face of the upper mandrel and an upper face of the lower mandrel contacting each other at an interface between them; moving the upper mandrel and the lower mandrel toward the lower punch while the upper and lower mandrels remain in contact with one another at the interface and transferring the interface to a predetermined position in the cavity charged with the powdered dielectric material; and compressing the powdered dielectric material in the cavity using relative movement between the upper punch and the lower punch while the upper mandrel and the lower mandrel remain in contact with each other at the interface, thereby forming the dielectric block.
In the method for press-molding a dielectric block according to a preferred embodiment of the present invention, the upper and lower mandrels are cylindrical in shape, r1 denotes the radius of the cylindrical lower mandrel, r2 denotes the radius of the cylindrical upper mandrel, and P denotes the distance of offset between the axis of the lower mandrel and the axis of the upper mandrel, and the expression 0xe2x89xa6Pxe2x89xa6r1+r2 is satisfied.
In the above-described transferring step, the powdered dielectric material is distributed in the cavity so as not to significantly apply pressure to the powdered dielectric material in the cavity and to form a shape of the dielectric block 1 and expanded in a direction of pressurization. Then, the density of the powdered dielectric material in the cavity is made substantially even by compressing the powdered dielectric material in the cavity. Therefore, cracks are not likely to be produced in the vicinity of a step part of a hole formed in the dielectric block, such as a dielectric filter or a dielectric duplexer.
The position of the lower mold and the position of the upper mold may be independently servo-controlled. With this arrangement, the position of the contact part between the upper and lower mandrels can be accurately controlled, whereby the density of the powdered dielectric material in the cavity can be more reliably made even.